1. Field of the Invention
The present invention relates to a communication method adapted to an image communication apparatus, and more particularly, to a data transmission protocol for an image communication apparatus which is capable of minimizing loss of data other than video and audio data and easily separating video, audio, and control data.
2. Description of the Background Art
FIG. 1 is a view illustrating the construction of an image communication apparatus according to the conventional art.
As illustrated therein, the image communication apparatus includes: a CCD 2 for converting an external light inputted via a lens 1 into an electric video signal; a CCD driving unit 3 for transferring the video signal converted by the CCD 2 to a digital signal processor(DSP) 4; a digital signal processor 4 for digitally processing the video signal inputted from the CCD driving unit 3 and outputting the same to be displayed on a monitor 7 in the format of a luminance signal and a color signal for image compression; an image compressing/restoring unit 5 for performing compression/restoration of the video signal; a communication control and interfacing unit 8 for communicating with communication lines and a voice input/output unit 9 for the purpose of transmitting the compressed video signal and receiving the transmitted video signal; and a controlling unit 6 for controlling each part of a system for the purpose of processing, compressing, restoring, and transmitting the above video signal. The operation of the thusly constructed image communication apparatus will now be described.
First, the external light inputted through the lens 1 is converted into an electric signal through the CCD 2 to be supplied to the CCD driving unit 3.
The CCD driving unit 3 corrects the converted video signal for an analog signal of a predetermined level, and thereafter outputs the same to the DSP 4. The DSP 4 performs clamping, RGB(Red Green Blue) color control, brightness control, etc. of the video signal outputted from the CCD driving unit 3, and thereafter outputs the same to the monitor 7 to display the image of the resultant video signal.
At this time, the DSP 4 converts the video signal outputted from the CCD driving unit 3 into a predetermined format of a luminance signal and a color signal to thus output the same to the image compressing/restoring unit 5 for the purpose of image compression and transmission to the outside.
The image compressing/restoring unit 5 executes compression in JPEG(Joint Photographic Experts Group) and MPEG(Moving Picture Expert Group) according to the type of the inputted video signal, and the compressed video signal is transmitted to the communication lines and the voice input/output unit 9 via the communication control and interfacing unit 8.
And, such a signal flow is controlled by the controlling unit 6.
As a data communication method utilizing a modem, i.e., the communication control and interfacing unit 8, in the image communication apparatus, there are a HDLC(High-Level Data Link Control) method and a NON-PROTOCOL method. The HDLC method is commonly being used. This will be explained with reference to FIG. 2.
FIG. 2 is a view illustrating a frame structure based on the HDLC method in case of modem data transmission according to the conventional art. That is, it is a frame structure for showing a data communication method utilizing the HDLC method.
As illustrated therein, in the HDLC method, a frame consists of a start flag(START Flag) representing the start of a frame, a data information(audio/video/control) to be transmitted, a CRC (Cyclic Redundancy Checking) code, i.e., an error detecting code, and a stop flag(STOP Flag) representing the stop of a frame.
In the above HDLC structure, the start and stop flag representing the start and stop of a frame contains a flag pattern of ‘01111110(7EHEC)’ as a combination of 8 bits for the purpose of frame synchronization. Such a flag synchronization method is a method for maintaining synchronization between a transmitter side and a receiver side all the time by sending a code of a predetermined pattern although there is no data to be transmitted. Here, the aforementioned predetermined pattern is called as the flag pattern of ‘01111110(7EHEC)’.
The receiver side, i.e., a modem, receiving such flags delivers data information to the controlling unit 6 after removing the start and stop flags.
In addition, a data information followed after the start flag and containing audio/video/control data is transmitted in a predetermined number of bytes. At this time, a data byte has a structure of 8 bits. However, a start bit and a stop bit are appended to both ends of the 8-bit structure, and thus the data byte is transmitted in 10 bits.
In addition, to search each of the audio/video/control data, a CRC code is contained in the data information itself.
In addition, only in case of a flag(01111110), a data stream has five consecutive 1's. Thus, in the case where original data information to be transmitted has six consecutive 1's, ‘0’(zero insert) is forcibly appended next to the fifth 1, whereby the start flag is distinguished from the stop flag and then the receiver side removes ‘0’ followed after five consecutive 1's.
Thereafter, the data stream is followed by the CRC code(error detection code), which is a combination of 16 bits, and checks whether there is an error or not in portions excepting the start and stop flags.
Meanwhile, a data communication method utilizing a modem, i.e., a communication control and interfacing unit 8 in the image communication apparatus includes the NON-PROTOCOL method. This will be explained with reference to FIG. 3.
FIG. 3 is a frame structure according to the NON-PROTOCOL method of the conventional art.
As illustrated therein, a data having a data stream alone is directly transmitted.
On the other hand, firstly, a transmission protocol for discrimination of files such as audio, video, control data, etc. of the above transmitted data information is established between the transmitter side and the receiver side, and then the transmitter side transmits data to the receiver side according to this protocol, for example, in the promise that the ratio of video to control is 3:2. Therefore, it is difficult for the receive side to discriminate files, and in the case where data transmission is carried out by changing the above transmission protocol within the range of a predetermined number of files, those files must be newly discriminated.
As described above, in the HDLC method according to the conventional art, a data information(audio/video/control/etc.) has another CRC code after acquiring the CRC code for error detection by using a modem line, so there is a problem that a redundancy occurs as much as the CRC code.
In addition, in the HDLC method according to the conventional art, although an overhead of a data byte, i.e., a data byte is 8 bits, 10 bits are actually transmitted in order to transmit the data byte because a start and a stop bit are appended upon transmission. Thus, data loss per second is increased, and the size of the data loss is also increased.
In addition, in the HDLC method according to the conventional art, ‘0’ is forcibly appended next to the fifth 1 in order to prevent the case where a data of a data information has five consecutive 1's is applied only to flags. That is, ‘0’ is inserted in order to prevent the case where the data contains the same data as the start/stop flags. Thus, there is a problem that the overhead of the data information is increased due to zero insertion and the receiver side must remove the inserted ‘0’.
In addition, in the NON-PROTOCOL method according to the conventional art, firstly, a transmission protocol for discrimination of files such as audio, video, control data, etc. of the above transmitted data information is established between the transmitter side and the receiver side, and then the transmitter side transmits data to the receiver side according to this protocol. Therefore, it is difficult for the receive side to discriminate files.
In addition, in the NON-PROTOCOL method according to the conventional art, in the case where data transmission is carried out by changing the above transmission protocol within the range of a predetermined number of files, those files must be newly discriminated.